Optical image scanners typically transfer scanned image data from scanning hardware to some type of device controller. Because of design and packaging constraints, the device controller may be located at a significant distance from the scanning hardware. This creates the potential to generate unacceptable amounts of electromechanical interference (EMI, also referred to as radio-frequency interference or RFI).
Various techniques can be used to limit or mitigate EMI emissions, including bypassing, decoupling, shielding, and clock dithering. When using clock dithering, which may also be referred to as PLL dithering, a communications clock and corresponding data transfer rate are modulated by a small amount, such as 0.5-5% of the nominal data transfer rate, with 3% being typical. This spreads emitted EMI through a range of frequencies, and results in a reduction or mitigation of measured EMI.
Clock dithering can typically be implemented without significant hardware costs such as might be involved with electrical and mechanical techniques. However, clock dithering can reduce the accuracy and resolution of image scanning. This is particularly true as designers attempt to achieve ever higher levels of scanner speed and performance.